1. Field of the Invention
The present invention relates to a reflective type liquid crystal display apparatus used in information display systems, office automation equipment, or the like, and to a front light for illuminating the same without deteriorating the display quality thereof.
2. Description of the Related Art
Typically, a reflective type liquid crystal display apparatus includes a reflective type liquid crystal display device (a liquid crystal panel) having a pair of glass substrates and a liquid crystal layer interposed therebetween. One of the glass substrates on the rear side of the reflective type liquid crystal display device is provided with a reflector. The other substrate on the light introduction side of the reflective type liquid crystal display device is provided with a polarization selecting section including a polarization plate, a quarter-wave plate, etc. While incident light transmitted through the polarization selecting section is reflected by the reflector, the polarization of the incident light is modulated by the liquid crystal layer. Thus, the amount of light which is output from the reflective type liquid crystal display device through the polarization selecting section is controlled, thereby displaying images.
Hereinafter, typical transitions of the polarization of light passing through this reflective type liquid crystal display apparatus will be described with reference to FIG. 14.
The reflective type liquid crystal display apparatus has a reflective type liquid crystal display device 65, in which a liquid crystal layer 66 is interposed between a pair of glass substrates 65a and 65b, and a reflector 67 is provided between the rear-side glass substrate 65b and the liquid crystal layer 66. On the front side of the reflective type liquid crystal display device 65, a polarizing plate 64a and a quarter-wave plate 64b are provided such that a transmission axis (or an absorption axis) of the polarizing plate 64a makes an angle of about 45xc2x0 with a slow axis (or a fast axis) of the quarter-wave plate 64b. 
A portion of incident light (linearly polarized light) transmitted through the polarizing plate 64a is converted to circularly polarized light by the quarter-wave plate 64b and incident upon the reflective type liquid crystal display device 65. When the liquid crystal layer 66 of the reflective type liquid crystal display device 65 does not modulate the incident circularly polarized light, the rotary direction of the circularly polarized light is inverted as the light is reflected by the reflector 67. Then, the light is again transmitted through the quarter-wave plate 64b (upwardly in FIG. 14) and is converted to linearly polarized light having a polarization direction A at about 90xc2x0 with the transmission axis of the polarizing plate 64a. Thus, the light is absorbed by the polarizing plate 64a, resulting in a black display.
On the other hand, when the liquid crystal layer 66 modulates the incident circularly polarized light, the light may be reflected by the reflector 67 and exit the reflective type liquid crystal display device 65 with the original circular polarization. Then, the light, after passing through the quarter-wave plate 64b again, becomes linearly polarized light whose direction of polarization B is identical with the direction of the transmission axis of the polarizing plate 64a so as to be transmitted through the polarizing plate 64a, resulting in a white display.
The directions of the transmission axis of the polarizing plate 64a and the slow axis of the quarter-wave plate 64b are determined in view of the liquid crystal material, the orientation of the liquid crystal material, the viewing angle characteristic, etc. Furthermore, in order to compensate for the tolerance of the phase delay with respect to the wavelength of the light output from the quarter-wave plate 64b, a half-wave plate may be provided between the polarizing plate 64a, and the quarter-wave plate 64b. The polarizing plate, the half-wave plate, and the quarter-wave plate are usually integrated together via adhesive layers, and further attached to the reflective type liquid crystal display device via another adhesive layer.
Furthermore, when images are displayed in colors by this reflective type liquid crystal display apparatus, light is transmitted through a color filter layer including color filter portions of three primary colors, i.e., red, green, and blue, which are provided in each pixel, whereby colored light can be obtained. Among various RGB arrangements, a delta arrangement as shown in FIG. 15A and a stripe arrangement as shown in FIG. 15B are commonly used. In these color filter arrangements, a unit pattern (corresponding to one pixel) comprising the three primary color portions is repeated in the vertical and horizontal direction.
Also, the number of pixels provided and the size of each pixel vary for different specifications. For example, for a 2.0xe2x80x3 reflective type liquid crystal display apparatus employing a delta arrangement, the number of pixels is 280xc3x97220 (horizontalxc3x97vertical), the pixel size is 145.5 xcexcm in the horizontal direction and 138.5 xcexcm in the vertical direction. For a 2.5xe2x80x3 display, the number of pixels is 280xc3x97220 (horizontalxc3x97vertical), and the pixel size is 179.5 xcexcm in the horizontal direction and 168.5 xcexcm in the vertical direction. For a 3.8xe2x80x3 QVGA reflective type liquid crystal display apparatus, pixels are patterned in a stripe arrangement, the number of pixels is 960xc3x97240 (horizontal pixelsxc3x97vertical pixels), and the pixel size is 81 xcexcm in the horizontal direction and 234.5 xcexcm in the vertical direction.
A reflective type liquid crystal display apparatus can display images using ambient light. However, since the brightness of the display is significantly dependent on the environment in which the apparatus is used, the displayed images may not be perceived at all, especially in the dark, e.g., at night.
Thus, for cases where sufficient ambient light cannot be obtained, a type of illuminator, called a xe2x80x9cfront lightxe2x80x9d, for illuminating a reflective type liquid crystal display device from the front side thereof, has been proposed in the art.
For example, in SID ""95 DIGEST, p.375, a front light as shown at reference numeral 81 in FIG. 16 is disclosed.
The front light 81 includes a light guide 83 and a light source 82 placed on an end surface of the light guide 83. An upper surface 83c opposite to a lower surface 83b of the light guide 83 includes periodic concave and convex portions. Light is output from the light source 82 and propagates through the light guide 83. The light may be reflected by the upper surface 83a one time so as to exit the light guide 83 or may be totally reflected by the lower surface 83b and/or the upper surface 83c several times while propagating through the light guide 83, so as to be reflected by the periodic concave-convex portions light guide 83 thereby exiting the lower surface 83b. Thereafter, the light output from the lower surface 83b is incident on a reflective type liquid crystal display device 85, to which the polarization selecting section 84 including a polarizing plate and a quarter-wave plate is attached via an adhesive layer 90.
However, when the above-described front light 81 is ON, some of the output light leaks through the upper surface 83c of the light guide 83 to directly reach the viewer""s eye, whereby the black display is degraded because of the light leakage. This significantly deteriorates the contrast of the display. Furthermore, the light leaking through the upper surface 83c may be incident on a foreign material such as dust in the vicinity of the upper surface 83c or a flaw on the surface of the light guide 83, which is then observed as a bright spot. In addition, the leak light may also be incident on a foreign material present between the front light 81 and the polarization selecting section 84, which is observed as a bright spot. These bright spots may deteriorate the production yield as well as the display quality of the device, and therefore are causes of serious problems.
Furthermore, the front light employing the above-described periodic concave/convex portions may generate two types of brightness fringe as follows.
The first type of brightness fringe will be described.
Light from the light source 82 is incident on the light guide 83 through the end surface 83a and propagates through the light guide 83. The light is then reflected by the periodic concave/convex portions of the upper surface 83c toward the lower surface 83b. A large portion of the light exits the lower surface 83b so as to irradiate the reflective type liquid crystal display device 85. However, about 4% of the light is reflected by the lower surface 83b and passes through the periodic concave/convex portions of the upper surface 83c so as to reach the viewer""s eye. Thus, since the viewer observes the light through the periodic concave/convex portions, the first brightness fringe is observed. Furthermore, for the same reason, the first brightness fringe is observed by the surface reflection of the polarization selecting section 84, thereby significantly deteriorating the display quality.
Next, the second type of brightness fringe will be described.
As described above, the light from the light source 82 is reflected by the periodic concave/convex portions of the upper surface 83c and exits the light guide 83 so as to be incident on the reflective type liquid crystal display device 85. The incident illumination light is reflected by each pixel of the reflective type liquid crystal display device 85. The reflected light is passed through the periodic concave/convex portions of the light guide 83 again so as to reach the viewer""s eye, whereby an image may be perceived by the viewer. Therefore, the second brightness fringe is observed as a result of the interference of light reflected by the periodic concave/convex portions of the light guide 83, light reflected by the pixel pattern of the reflective type liquid crystal display device 85, and light transmitted through the periodic concave/convex portions of the light guide 83. Likewise, in the case where images are observed with ambient light, light passes through the periodic concave/convex portions of the light guide 83, the pixel pattern of the reflective type liquid crystal display device 85, and the periodic concave/convex portions of the light guide 83 again, thereby generating the second brightness fringe due to interference.
Japanese Laid-Open Utility Model Publication No. 63-4515 discloses a method for preventing the reflection at the interface between the light guide and the liquid crystal panel. In this method, a liquid crystal cell 31 and a light guide 35 are closely attached together via a transparent resin 36, as shown in FIG. 17. However, this utility model publication does not disclose components such as a polarizing plate, and a phase plate. Thus, it does not disclose any of the effects provided by the present invention in which the polarization selecting section is attached to the light guide of the front light.
Furthermore, when the liquid crystal cell 31 and the light guide 35 are closely attached together as shown in FIG. 17, since they are both rigid, bubbles may be introduced therebetween, an adhesive resin may not be sufficiently cured, or a re-work may not be done. Thus, the production of the device may become difficult.
According to one aspect of the present invention, a front light includes a light source, a light guide for receiving light from the light source through an end surface of the light guide and outputting the light through a first large surface which is substantially perpendicular to the end surface, and a polarization selecting section for selectively transmitting light having particular polarization, the polarization selecting section being attached to the first large surface of the light guide such that any reflection does not occur at an interface between the polarization selecting section and the light guide.
In one embodiment of the present invention, the polarization selecting section is attached to the light guide via a refraction layer having a refractive index approximate to refractive indices of the light guide and the polarization selecting section.
In another embodiment of the present invention, the refractive index of the refraction layer is slightly different from the refractive index of the light guide.
In still another embodiment of the present invention, the refractive index of the refraction layer is smaller than the refractive index of the light guide.
In still another embodiment of the present invention, a difference between the refractive index of the refraction layer and the refractive index of the light guide is more than 0 and is 0.2 or less.
In still another embodiment of the present invention, a second large surface opposite to the first large surface of the light guide includes periodic concave/convex portions formed at a predetermined pitch, each of the periodic concave/convex portions including a propagation portion for propagating light from the light source and a reflection portion for reflecting the propagating light toward the first large surface.
In still another embodiment of the present invention, the polarization selecting section is a combination of a polarizing plate and a quarter-wave plate.
In still another embodiment of the present invention, the polarization selecting section is a combination of a polarizing plate, a half-wave plate, and a quarter-wave plate.
According to one aspect of the present invention, a reflective type liquid crystal display apparatus includes a front light according to claim 1, and a reflective type liquid crystal display device including a plurality of pixels for receiving light from the first large surface of the light guide through the polarization selecting section, controlling polarization of the received light for each of the pixels, and reflecting the polarized light toward the polarization selecting section.
In one embodiment of the present invention, a surface of the polarization selecting section facing the reflective type liquid crystal display device is subjected to an anti-reflection treatment.
In another embodiment of the present invention, a surface of the reflective type liquid crystal display device facing the polarization selecting section is subjected to an anti-reflection treatment.
In still another embodiment of the present invention, the reflective type liquid crystal display device and the polarization selecting section are not in contact with each other.
In still another embodiment of the present invention, the periodic concave/convex portions are formed such that a direction of a stripe thereof is not identical with a horizontal direction of a repetitive pixel pattern of the reflective type liquid crystal display device.
In still another embodiment of the present invention, the periodic concave/convex portions are formed such that the direction of the stripe thereof makes an angle of about 10xc2x0 to about 80xc2x0 with the horizontal direction of the repetitive pixel pattern.
In still another embodiment of the present invention, the pixel pattern of the reflective type liquid crystal display device is a delta arrangement, and the periodic concave/convex portions are formed such that the direction of the stripe thereof makes an angle of about 10xc2x0 to about 25xc2x0 or an angle of about 55xc2x0 to about 80xc2x0 with the horizontal direction of the repetitive pixel pattern.
In still another embodiment of the present invention, the pixel pattern of the reflective type liquid crystal display device is a stripe arrangement, and the periodic concave/convex portions are formed such that the direction of the stripe thereof makes an angle of about 15xc2x0 to about 75xc2x0 with the horizontal direction of the repetitive pixel pattern.
Hereinafter, functions of the present invention will be described.
According to the present invention, a polarization selecting section is optically attached to the light guide such that the reflection does not occur at the interface therebetween. A light incident on an end surface of the light guide propagates through the light guide. During its propagation, the light reaches a second large surface of the light guide, i.e., the light reaches one of periodic concave/convex portions of an upper surface which is opposite to a first large surface (a lower surface) of the light guide, and the light is reflected by one of periodic concave/convex portions toward the first large surface. Only light having particular polarization is selectively transmitted through a polarization selecting section so as to irradiate the reflective type liquid crystal display device. The reflective type liquid crystal display device controls the polarization of the irradiated light for each pixel by the liquid crystal layer and reflects the irradiated light toward the reflective type liquid crystal display device. Thereby, images are displayed while controlling the amount of light transmitted through the polarization selecting section.
The light propagating through the front light is transmitted through the polarization selecting section repeatedly during its propagation, so that light other than the light having particular polarization is absorbed by the polarizing plate. Therefore, in the front light of the present invention, as compared with the conventional front light in which light propagating through the light guide is not polarized in any direction, the amount of light is reduced by xc2xd or more, whereby the amount of light which leaks from the upper surface of the light guide toward a viewer is reduced to xc2xd or less. Thus, the deterioration of the display contrast can be suppressed. Furthermore, for the same reasons, bright spots resulting from foreign materials, on which leak light is incident and scattered, can be reduced, thereby improving the display quality and the production yield.
Furthermore, according to the present invention, light is transmitted through the polarization selecting section attached to the light guide before it exits the front light so as to be converted to light having particular polarization which is necessary for the display of the reflective type liquid crystal display device. On the other hand, in the conventional front light, light output from the front light is transmitted through the polarization selecting section attached to the reflective type liquid crystal display device such that only light having particular polarization which is necessary for the display is transmitted through the polarization selecting section. Therefore, in the front light of the present invention, the amount of light is xc2xd or less than that of the conventional front light while the amount of the polarized light incident on the reflective type liquid crystal display device is equal to that in the conventional front light. As a result, bright spots resulting from foreign materials, on which the light from the front light is incident and scattered, can be reduced, thereby improving the display quality and the production yield.
According to the present invention, the polarization selecting section is attached to the light guide through a refraction layer whose refractive index is close to both of the refractive indices of the light guide and the polarization selecting section. Thereby, the light guide and the polarization selecting section can be optically integrated together such that any reflection does not occur at the interface therebetween.
However, in the front light, it is required that light introduced from the light source and incident on an end surface of the light guide propagate through the light guide by utilizing the reflections on the first and second large surfaces. Therefore, when a refractive index of the light guide is completely equal to that of the refraction layer so that no reflection occurs on the first large surface, propagation of the light is hindered, whereby the intensity of the light decreases as it goes away from the light source. Thus, it is preferable that a refractive index of the refraction layer provided between the light guide and the polarization selecting section be slightly different from that of the light guide. In Embodiment 3 which will be described later, as shown in FIG. 11, by providing a refractive layer having a refractive index slightly different from that of the light guide, light which is incident upon the interface at a large angle is reflected by the interface, whereby the amount of light propagating through the light guide increases. Thus, such reflection at the interface effectively promotes the propagation of the light through the light guide, thereby suppressing the decrease of the intensity of the illumination light so as to obtain uniform illumination light. Furthermore, since light which is incident upon the interface at a large angle is reflected by the interface, and is not reflected toward the viewer, the display quality does not deteriorate.
Specifically, it is preferable that the refractive index of the refraction layer be regulated so as to be smaller than that of the light guide because a total reflection (where the reflectivity is 1) occurs in this condition, whereby the light effectively propagates through the light guide.
Furthermore, in Embodiment 3 which will be described later, as shown in FIG. 12, it is preferable that the difference in the refractive index between the light guide and the refractive layer be in the range from about 0 to about 0.2, because the surface reflection increases as the difference increases. When the difference in the refractive index between the light guide and the refractive layer is about 0.2 or less, a reflectivity at the interface therebetween is about 0.5% or less. Therefore, the reflectivity at the interface between the light guide and the reflection layer, and the reflectivity of the interface between the refraction layer and the polarization selecting section can be reduced to about 1% or less.
When the polarization selecting section is attached to the light guide via a refraction layer whose refractive index is close to both of refractive indices of the light guide and the polarization selecting section, the polarization selecting section can be attached to the light guide via an adhesive layer having a refractive index of, for example, about 1.5. Furthermore, by regulating the refractive index of the adhesive layer so as to be slightly different from that of the light guide as described above, uniform illumination can be obtained. Alternatively, the polarization selecting section may be attached to the light guide through a transparent film having a refractive index approximate to both that of the light guide and that of the polarization selecting section, for example, about 1.5. The polarization selecting section may alternatively be attached to the light guide via a transparent resin. Also in these cases, by regulating the refractive index of the transparent film or the transparent resin so as to be slightly different from that of the light guide, uniform illumination can be obtained.
In the conventional structure, the upper surface of the light guide may include periodic concave/convex portions which are formed at a predetermined pitch, wherein each of the periodic concave/convex portions includes a propagation portion through which light from the light source propagates, and a reflection portion which reflects the incident light toward a lower surface. In such a structure, light reflected by the lower surface of the light guide or the surface of the polarization selecting section reaches the viewer""s eye through the periodic concave/convex portions, whereby the above-described first brightness fringe is observed. In the present invention, on the other hand, the polarization selecting section is attached to the lower surface of the light guide. Therefore, the reflection at the interface between the light guide and the polarization selecting section can be reduced to improve the display quality. Furthermore, when the polarization selecting section is attached to the light guide via a refraction layer having a refractive index slightly different from that of the light guide, the surface reflection is small because the refractive index difference therebetween is very small. As described above, by regulating the difference in refractive index between the light guide and the polarization selecting section so as to be about 0.2 or less, the surface reflection is reduced to about 1% or less, thereby preventing the first brightness fringe from being observed.
As described above, in the front light where the upper surface of the light guide includes periodic concave and convex portions, light passes through the periodic concave/convex portions of the light guide, a pixel pattern of the reflective type liquid crystal display device, and again the periodic concave/convex portions of the light guide before it reaches the viewer""s eye, whereby the second brightness fringe may be observed. In the present invention, in order to prevent the second brightness fringe from being observed, the periodic concave/convex portions are designed so that the direction of the stripe of periodic concave/convex portions is not identical with the direction of a repetitive pixel pattern. In such a structure, a pitch of the second brightness fringe is shortened so that it is not perceived by the viewer, whereby the display quality can be improved.
When the pixels are patterned in a delta arrangement, it is preferable that the stripes of the periodic concave/convex portions be designed so as to make an angle in the range from about 10xc2x0 to about 25xc2x0 or from about 55xc2x0 to about 80xc2x0 with the horizontal direction of the repetitive pixel pattern. In this structure, the second brightness fringe is not observed, as will be later described in Embodiment 1 with reference to FIG. 7.
When the pixels are patterned in a stripe arrangement, it is preferable that the stripes of the periodic concave/convex portions be designed so as to make an angle in the range from 15xc2x0 to 75xc2x0 with the horizontal direction of the repetitive pixel pattern. In this structure, the second brightness fringe is not observed, as will be later described in Embodiment 1 with reference to FIG. 8.
The polarization selecting section may be a plate including a polarizing plate and a quarter-wave plate optically attached to each other can be used. When the polarization selecting section is designed such that an angle between the transmission axis (or the absorption axis) of the polarizing plate and the slow axis (or the fast axis) of the quarter-wave plate is about 45xc2x0, only circularly polarized light which has passed through polarizing plate and the quarter-wave plate is output from the front light as illumination light. While the circularly polarized light is incident on the reflective type liquid crystal display device and reflected by the reflector thereof, the polarization of the light is modulated so that the amount of light which is transmitted through the polarizing plate is controlled.
Alternatively, the polarization selecting section may be a plate further including a half-wave plate optically attached between a polarizing plate and a quarter-wave plate. In this structure, the half-wave plate can compensate for the tolerance of the phase delay with respect to the light output from the quarter-wave plate, whereby circularly polarized light, which is suitable for the reflective type liquid crystal display device, can be obtained over a wide wavelength range.
By employing such structures for the polarization selecting section, a portion of circularly polarized light which has passed through the polarization selecting section, i.e., light reflected by a surface of the front light facing the reflective liquid crystal display device (i.e., an interface between the quarter-wave plate and air) and light reflected by the surface of the reflective liquid crystal display device are converted to linearly polarized light crossing the transmission axis of the polarizing plate with an angle of 90xc2x0 when the reflected light are transmitted through the quarter-wave plate again, so as to be absorbed by the polarizing plate. Therefore, the brightness fringe of the light guide can be prevented from being perceived, and the deterioration of the contrast ratio of the display due to the surface reflection can be prevented.
In the reflective liquid crystal display apparatus of the present invention, about 4% surface reflection is generated on the interface between the front light and the reflective liquid crystal display device, i.e., on the surface of the polarization selecting section, and on the surface of the reflective liquid crystal display device. Light beams resulting from this surface reflection are absorbed by the polarizing plate of the polarization selecting section. Thus, this surface reflection does not affect the contrast ratio of the display. However, due to this surface reflection, the amount of light incident on the reflective liquid crystal display device and used for the display, and the amount of light reflected by the reflective liquid crystal display device to reach the viewer""s eye decrease, thereby deteriorating the brightness of the display. Therefore, it is preferable that a surface of the polarization selecting section facing the reflective liquid crystal display device, or a surface of the reflective liquid crystal display device facing the polarization selecting section, or both of these surfaces be subjected to the anti-reflection treatment.
In the present invention, since it is undesirable that the reflective liquid crystal display device and the front light are in close contact with each other, the reflective liquid crystal display device is not attached to the front light. That is, when the reflective type liquid crystal display device (a liquid crystal cell) and the light guide are closely attached to each other as in the conventional display apparatus, since they are both rigid, bubbles maybe introduced therebetween, an adhesive resin may not be sufficiently cured, or a re-work may not be done, whereby the production of the device becomes difficult. On the other hand, in the present invention, an air layer is present between the reflective liquid crystal display device and the front light. However, light reflected by the interfaces thereof is absorbed by the polarization selecting section, as described above, and therefore does not affect the contrast ratio of the display. Furthermore, the reflection of light can be prevented by the anti-reflection treatment described above.
Thus, the invention described herein makes possible the advantages of providing (1) a front light which can effectively output light without deteriorating the display quality and (2) a reflective type liquid crystal display apparatus with high brightness and high production yield.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.